Although the annual production of gold in the United States has been increasing steadily, oxide gold ore reserves are rapidly diminishing. In this regard, more complex ores, including refractory gold ores and low-grade ores, are being processed. In many cases, these resources contain significant amounts of mercury along with the gold values. Generally, the extraction of gold is accomplished by cyanidation in which leaching occurs by the addition of cyanide at alkaline pH. Cyanide, which is a strong lixiviant for gold, is an equally strong lixiviant for mercury. According to Sharpe (Sharpe, A. G., "The Chemistry of Cyano Complexes of the Transition Metals," (London Academic Press) 1976), simple and complex cyanides of mercury are restricted to the oxidation state of 2 and dissolve as the Hg(CN).sub.2.degree., and HG(CN).sub.3.sup.-1 and HG(CN).sub.4.sup.-2 complexes of mercury.
Typically mercury (II) is released as cyano complexes during leaching and follows gold through the solution concentration and purification steps. Purification is done by carbon adsorption (Ibrada, A. S. and Fuerstenau, D. W., "Adsorption of the Cyano Complexes of Ag(I), Cu(I), Hg(II), Cd(II) and Zn(II) on Activated Carbon," Minerals and Metallurgical Processing (Feb. 1989) pp. 23-28) and electrowinning or zinc cementation (Sandberg, R. G. et al., "Calcium Sulfide Precipitation of Mercury During Cyanide Leaching of Gold Ores," (1984), RI8907, USBM).
In a typical cyanidization carbon-in-pulp (CIP) process (Sandberg, R. G., et al. (1984) "Calcium Sulfide Precipitation of Mercury During Cyanide Leaching of Gold Ores," RI 8907 USBM), the ore is ground with lime, adding water to form a slurry, and sodium cyanide is added to solubilize the gold. The mixture is thickened using various methods of flocculation, and the underflow is treated with calcium sulfide to precipitate mercury as HgS. The slurry is then exposed to activated carbon to adsorb gold and silver. Mercury is also adsorbed onto the carbon and interferes with gold and silver recovery. Precipitated mercury sulfide reports with tailings where it forms a disposal problem. U.S. Pat. No. 4,289,532 to Matson et al. also discloses a typical gold cyanidation process.
The overflow from the thickening process may also be subjected to mercury precipitation and filtration to recover mercury sulfide, and the filtrate exposed to activated carbon for removal of gold, silver and unprecipitated mercury. The loaded carbon streams exiting from the activated carbon treatments are stripped using a concentrated heated caustic cyanide strip solution and gold and silver recovered by electrowinning. The presence of mercury during exposure to activated carbon also interferes with recovery of the precious metals.
The presence of mercury in all steps of cyanidation poses a threat to both the environment and to the health of the plant workers. The presence of high concentrations of mercury (&gt;50 g/t) has been shown to reduce significantly the efficiency of gold cementation with zinc (Marsden, J. and House, I., "The Chemistry of Gold Extraction (Great Britain, Ellis Horwood Limited), 1992, p. 397). Mercury vapor can be released during carbon stripping, carbon regeneration, electrowinning, and retorting (Staker, W. L. et al., "Mercury Removal from Cold Cyanide Leach Solutions," Gold and Silver: Heap and Dump Leaching Practice, 1984, p. 119). Further, in the CIP process, mercury builds up in the recycled leach solutions and reports to the tailings because only part of the mercury is adsorbed on carbon in the loading circuit (Sandberg, R. G. et al., "Calcium Sulfide Precipitation of Mercury During Cyanide Leaching of Gold Ores," RI 8907 USBM, 1984) Thus the tailings impoundment can . also represent an environmental concern.
Inorganic mercury can be transformed into methylmercury and dimethylmercury by the action of microorganisms under aerobic conditions and is favored by alkaline conditions. These compounds are volatile and can be released to the atmosphere (Organization for Economic Cooperation and Development, "Mercury and the Environment: Studies of Mercury Use, Emission, Biological Impact and Control," (Organization for Economic CoOperation and Development, Paris) 1974, p. 37). Furthermore, the methylation of mercury by sulfide reducing bacteria, producing a form of mercury that is more toxic and biologically available to biota, has been demonstrated (Porcella, D., "Mercury in the Environment: Biogeochemistry," Mercury Pollution, Integration and Synthesis, (C. Watras, J. Huckabee, Eds.) U.S.A., Lewis Publisher, (1994) p. 3).
It is evident that the problem of mercury in the cyanidation of gold and silver ores is a two-fold problem involving: (1) the presence, and possible volatization of mercury from post-leaching process streams which presents environmental and health problems; and (2) the presence of mercury, in its various forms, within the tailings impoundment and recycled process water. Methods to control mercury during gold cyanidation are of particular interest to the gold mining industry. In this regard, a number of different chemical processes for mercury control have been devised. These include precipitation, adsorption, solvent extraction, ion exchange and cementation. For example mercury may be selectively removed from solution by ion exchange resins (Staker, W. L. and Sandberg, R. G., "Selective Elution of Mercury, Silver and Gold from Strong-Base Anion Exchange Resins," (1986) RI9093, USBM). Solvent extraction has also been used (Diaz, X. et al., "Selective Solvent Extraction of Gold from Mercury in Concentrated Alkaline Cyanide Solutions," Minerals, Metals and Materials Soc. (1993) 245-257 EPD Congress 1993, 1992). Flocculating agents may be used to enhance mercury removal (U.S. Pat. No. 4,726,939 issued Feb. 23, 1988 to Touro for "Process for the Removal of Mercury from Precious Metal-Cyanide Liquors"). Another method of precipitating mercury from cyanide leach solutions involves the use of polysulfides such as calcium polysulfide. Although information is limited on the chemistry and mercury-precipitating potential of polysulfides, they have been used in at least one commercial application. Ultimately, prior to the production of ore metal, elemental mercury is removed by retorting.
Organic compounds such as water-soluble polymers recover mercury and other heavy metals from solution (U.S. Pat. No. 4,619,744 issued Oct. 28, 1986 to Horton for "Recovery of Heavy Metals from Aqueous Solutions"). Trithiocarbanates have also been used for this purpose (U.S. Pat. No. 4,678,584 issued Jul. 7, 1987 to Elfine for "Method of Removing Heavy Metal From Wastewater Streams"). Thiourea has been used to recover gold and mercury from solution (U.S. Pat. No. 4,681,628 issued Jul. 21, 1987 to Griffin et al. for "Gold Recovery Process"). None of these processes are selective for the recovery of mercury from solutions also containing gold.
Selective stabilization and/or removal of mercury has been accomplished by precipitation with inorganic sulfides (U.S. Pat. No. 4,734,270 issued Mar. 29, 1988 to Touro for "Sulfide Treatment to Inhibit Mercury Adsorption onto Activated Carbon in Carbon-in-Pulp Gold Recovery Circuits"). The gold cyanide complex is stable with respect to sulfide precipitation in alkaline cyanide solutions and on this basis a selective separation is possible. Studies have been reported using H.sub.2 S, Ag.sub.2 S, FeS, Na.sub.2 S, and CaS. Sandberg et al. at the U.S. Bureau of Mines in Salt Lake City, Utah have reported on mercury precipitation from gold cyanide leach solutions with CaS and Na.sub.2 S (Sandberg, R. G. et al. (1984), "Calcium Sulfide Precipitation of Mercury During Cyanide Leaching of Gold Ores," RI 8907 USBM; Staker, W. L. and Sandberg, R. G. (1986), "Calcium Sulfide Precipitation of Mercury for gold-Silver Leach Slurries," RI 9042, USBM). They found that Na.sub.2 S and CaS can serve as precipitants for mercury but report that redissolution of mercury occurs when Na.sub.2 S is used. Calcium sulfide, on the other hand, is reported to minimize the rate of mercury redissolution but, like all other sulfides, tends to precipitate silver from solution as well.
Known methods of removal of mercury from process solutions suffer from one drawback or another. For example, the ion exchange resins are expensive and not very selective. The use of cementation by addition of noble metals has a detrimental effect and also contributes to gold loss. Selective solvent extraction using dibutylbutylphosphonate is unsatisfactory because it applies to strip solutions prior to the electrowinning circuit. In this case the mercury has already had a chance to volatize during the stripping and carbon regeneration circuit.
The only feasible known method for mercury recovery from gold cyanide circuits is the use of CaS and Na.sub.2 S. H.sub.2 S, AgS and FeS can also be used, but H.sub.2 S is not recommended due to its high toxicity, while the use of FeS would form ferrocyanide in leach solutions, tying up vast amounts of cyanide, such that its use would be prohibitively expensive (Sandberg, R. G. et al., "Calcium Sulfide Precipitation of Mercury During Cyanide Leaching of Gold Ores," RI 8907 USBM, 1984). AgS would also not be economical to use due to its costly silver component. However, the sulfides of HgS tend to resolubilize in cyanide solution after a few hours and are not stable at high cyanide concentration. CaS and Na.sub.2 S are able to precipitate mercury only from the Hg(CN).sub.4.sup.-2 complex and cannot efficiently precipitate it from the dicyano complex, Hg(CN).sub.2. Moreover, mercury redissolves when Na.sub.2 S is used, probably due to the fact that Na.sub.2 S is useful only in a limited pH range below 10 in which cyanide tends to dissociate into HCN. CaS is reported to minimize the rate of mercury redissolution but, like all other sulfides, tends to precipitate silver from solution as well. (However it was found that the addition of copper into solution deters silver precipitation while having no effect on the mercury precipitation rate (Staker, W. L. and Sandberg, R. G., "Calcium Sulfide Precipitation of Mercury from Gold-Silver-Leach Slurries," (1986) RI9042, USBM)).
A method is needed to selectively remove mercury from process solutions, particularly cyanide-containing solutions also containing gold and silver, as found in gold recovery processes, such that the mercury does not redissolve in the solution.
Dialkyldithiocarbamates are known to the art and potassium dimethyldithiocarbamate is known to form insoluble metallic salts except with those elements of the alkali and alkaline earth families (Leja, J., "Flotation Surfactants," Surface Chemistry of Froth Flotation (1982) Plenum Press, New York, pp. 258-259). Complexes formed between these compounds and various metals are also known (Galvez, J. et al., "Catalytic Hydrogen Wave in Presence of Dimethyldithiocarbamate and Cobalt II," Electrochemical Acta (1984) 29:253-256; Bond, A. M. et al., "Electrochemical Investigation of Kinetic and Thermodynamic Aspects of Oxidation and Reduction of Mononuclear and Binuclear Rhodium Dithiocarbamate and Diselenocarbamate Complexes," Inorg. Chem. (1989) 28:54-59; Stanislav, M. and Vladimir, F., "Theoretical study of the electron structure and properties of dithiocarbamates and their complexes with transition metals," Collect. Czech Chem. Commun. (1984) 49:2744-2750, Chem. Abstracts 1985 Vol. 102, No. 100934a). Mercury complexes with such organic compounds have been mentioned in the literature (Wan, Q. et al., "High-Performance Liquid Chromatographic Determination of Aliphatic Secondary Amines as Mercury (II) Chelates," Chem Abstracts (1986) Vol. 105, No. 53850v). U.S. Pat. No. 3,561,946 issued Feb. 9, 1971 to Braxton et al. discloses inter alia alkyl mercury complexes with dimethyldithiocarbamate as useful herbitoxic compositions.
All patents and publications mentioned herein are incorporated by reference.